Superparamagnetic Iron Oxide–Based Method for Quantifying Recruitment of Monocytes to Mouse Atherosclerotic Lesions In Vivo

Abstract

Background-It has been found recently that the MRI contrast agent superparamagnetic iron oxide (SPIO) localizes to aortic atherosclerotic plaques. We therefore asked whether SPIO might be used to monitor monocyte recruitment into aortic atherosclerotic plaques. Methods and Results-Eleven female apo E knockout (K/O) mice, each 11 months old, were divided into 2 groups. Six mice received tissue necrosis factor-␣ (0.2 g IP once), interleukin-1␤ (0.2 g IP once), and interferon-␥ (100 U/g per day IP for 5 days); 5 received 0.5 mL saline containing1% BSA and served as sham-treated atherosclerotic controls. Two wild-type C57BL/6 mice served as sham-treated nonatherosclerotic controls. Three hours after initial cytokine or sham treatment, all mice received SPIO by intravenous injection (1 mmol/kg iron). Six days later, all mice were euthanized, the hearts and aortas were perfused under physiological pressure, and the entire aortas were studied histologically. Atherosclerotic plaques in cytokine-treated mice contained more iron-positive macrophages per cross section than did those in sham-treated apo E K/O control mice (42Ϯ11.8 versus 11.6Ϯ5.9) (PϽ0.0001). Iron-laden macrophages were present either in subendothelial plaque surfaces or in thin layers overlying the internal elastic lamina, often at the edges of atherosclerotic plaques. No iron deposition was seen in aortas of the wild-type nonatherosclerotic control mice. Immunocytochemistry showed mostly macrophages and few T lymphocytes in atherosclerotic plaques of cytokinetreated mice. Key Words: atherosclerosis Ⅲ imaging Ⅲ magnetic resonance imaging Ⅲ leukocytes Ⅲ interleukins A therosclerosis is an inflammatory disease. 1 Inflammatory cells play a pivotal role in its initiation and progression. Increased macrophage density and/or activation in atherosclerotic plaque and the subsequently increased secretion of matrix metalloproteinases and possibly other proteases may induce the breakdown of collagen in the fibrous cap, thus contributing to the plaque's vulnerability to rupture. 2,3 Finding a noninvasive method for detecting monocyte recruitment into atherosclerotic plaques is a topic of great interest in atherosclerosis research. 4 -7 Such a technique could be used to assess plaque initiation, progression, and complications. Our group has been interested in the detection of inflammation in plaques by MRI 5 to 7 days after intravenous administration of superparamagnetic iron oxide (SPIO). SPIO is a nanoparticle that is avidly taken up by the reticuloendothelial system. 8 In studies performed in Watanabe heritable hyperlipidemic rabbits (transgenic rabbit in which atherosclerosis develops spontaneously), Schmitz et al 9 have shown that SPIO also homes to aortic atherosclerotic plaques, thus suggesting a potential noninvasive means for assessing atherosclerotic plaque. Conclusions-SPIOCytokines have long been known to enhance the recruitment of monocytes into atherosclerotic lesions. 6,10,11 The purpose of the present study was to compare the uptake of iron (in the form...

p38 MAPK Inhibition Reduces Aortic Ultrasmall Superparamagnetic Iron Oxide Uptake in a Mouse Model of Atherosclerosis

“…Additionally, superparamagnetic iron oxide (SPIO) has been used to quantify monocyte recruitment histologically in atherosclerotic lesions of the apolipoprotein E-deficient (apoE Ϫ/Ϫ ) mouse. 15 In that study, acute administration of tumor necrosis factor (TNF), interleukin (IL)-1␤, and interferon (INF)-␥ was shown to stimulate SPIO uptake by 4-fold in plaque. Although this study supports the hypothesis that plaque targeting of the USPIO contrast agent in mouse is facilitated by monocyte recruitment or activation of macrophage phagocytosis, noninvasive assessment of USPIO uptake in mouse atherosclerotic lesions has not been reported.…”

Detection and Treatment of Vulnerable Plaques and Vulnerable Patients

“…MRI capabilities may be enhanced with the use of agents to enhance contrast of macrophage presence. 62 Novel agents have been developed for MR (and near-infrared [NIR]) molecular imaging that are activated by local enzymatic processes. 63 Strauss et al 64 have noted that the ability of nuclear studies to detect metabolic activity may eventually lead to successful noninvasive studies of vulnerable coronary artery plaques.…”

Patterns of USPIO Deposition in Murine Atherosclerosis

“…This is a general observation, and again it must be stressed that many factors can and will influence the pattern as well as quantity of USPIO particle uptake by plaque including, animal model, plaque location, and USPIO dosing paradigm. In another study examining Feridex deposition in atherosclerotic plaque of apoE Ϫ/Ϫ mice treated with a proinflammatory cytokine cocktail, Litovsky et al 7 showed iron deposition to be largely intracellular and associated with subendothelial macrophages as well as occasionally found in deeper regions adjacent to the internal elastic lamina. We have also found general colocalization of the Perl's staining with regions associated with macrophages ( Figure, D).…”